You will then need to Add and Reference System.Web in your test project. Once done, you will be able to set your HttpContext and set HttpContext specific values, such as session variables. In the example below, I am setting up the HttpContext in the SetUp method of a unit test:

Another more heavy solution to the above would be to use a factory to get at and create your session. I opted for the solution above as it meant not changing my application code to fit in with my unit tests.